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Bmc alternator requiring a rev up


Monkey 1

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When I start my bmc 1.5, I have to give it quite a bit of throttle before the red charge light goes out. Is this normal? I've never met anyone else who has to do this!

 

I have replaced the alternator with a new one after the old one failed but it still requires a high rev. Also, the new alternator has a loose blue wire coming out of the casing? Any idea what that is?

 

I'm sure this is not good practice on a cold engine so I normally let it warm up a while first, but it is a bit of a pain.

 

Your thoughts are appreciated!

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When I start my bmc 1.5, I have to give it quite a bit of throttle before the red charge light goes out. Is this normal? I've never met anyone else who has to do this!

 

I have replaced the alternator with a new one after the old one failed but it still requires a high rev. Also, the new alternator has a loose blue wire coming out of the casing? Any idea what that is?

 

I'm sure this is not good practice on a cold engine so I normally let it warm up a while first, but it is a bit of a pain.

 

Your thoughts are appreciated!

 

If you have a split charge relay, it requires an electrical impulse from the alternator to activate it. This is not normally a problem if the engine is still warm as there is presumably some residual power in the system, but when the engine is cold it often requires a quick rev to produce the impulse. If this bothers you, let the engine run for a minute or so until the oil has circulated and then rev it. It is revving an engine with comparatively dry bearings which may cause damage, so once the oil is circulating there should not be a problem.

 

The "loose wire" coming from the casing is probably a field wire which is used to activate an external alternator regulator such as a Sterling. Different suppliers use different colours, I believe the industry standard is supposed to be white, but they are green on the alternators I have (I have one on the engine , and two spares !!).

Edited by David Schweizer
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Although nothing to do with a split charge relay its not unusual, especially if the pulley ratio from engine crankshaft to alternator is a little low, to have to rev the engine a little before the field winding is self excited, enabling alternator to produce output current. Until this happens the warning light will remain on.

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yep same here. My original alternator needed a blip on the throttle to put the charge light out. I have fitted a new higher rated alternator and the light still doesn't go out straight away. I can blip the engine which will extinguish it but the light will go off after a minute or so of the engine running. I also have a BMC 1.5 engine.

 

Pete

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I've been doing a little research on this alternator business and I came across a porsche forum where an owner had a similar issue. Apparently on his model there is a resistor across the alternator warning light fooling the alternator into thinking the bulb uses more power, so the alternator charges at start up.

 

So, I have just replaced my 5w alternator warning light bulb with a 20w car indicator bulb and hey presto, the light extinguishes as soon as the engine is running and the alternator charges with no revs at all.

 

This has to be better than reving my 40 year old bmc hard every day to get it charging! Happy man

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Woa! Easy cowboy! 20W! You could be getting perilously close to pulling in a split charge relay prematurely there.

Do we need to explain that point?

 

Typically your boat will probably have a split charge relay. This is to ensure that the domestic and engine batteries are separated when the engine is stopped, but connected together when the engine is running so both get charged. The relay just connected the +ves of the domestic and engine batteries together. The relay coil is typically connected between the alternator warning light connection on the alternator, and ground. Thus whilst the alternator is not generating charge (the terminal near 0v, just elevated slightly by the warning light) the relay isolates the two batteries. Once the alternator starts charging the voltage on the alternator warning light terminal goes up to 13v so so and thus the relay operates, connecting the battery banks together.

 

However if the bulb is too high a wattage, the voltage on the alternator warning light terminal will be raised significantly, possibly to the point at which the relay operates. Thus the relay operates as soon as the "ignition" is switched on. This means that when you operate the starter, the relay may already be connected and thus very high starting currents may flow through the relay, well above its current rating.

 

Therefore check that the relay doesn't operate when you turn the ignition on, or preferably fit a "compromise" bulb of just a few watts.

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Thanks for the info. Ok so I need to use a lower wattage bulb then? I did wonder. My 20w bulb was just a trial, hanging out of the control panel on the end of a wire!

 

I think what you say IS happening because the bulb extinguished as soon as the starter is cranked, before the engine has fired.

 

I shall buy a selection of bulbs and work my way up the watts until the light only extinguishes once running. Does that sound about right?

 

One thing I don't understand is if the voltage supply to the warning light increases once the alternator is charging, how does it extinguish?

Edited by Monkey 1
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My old alternator needed periodic tensioning as it wasn't lined up perfectly. Last year I had to re wire the engine as I hadn't noticed after adjusting the alternator, that the case was touching the +ve terminal on the glow plug above it! One cold morning after a lenthly pre heat It melted the loom filling the boat with smoke.

 

I ensured this new one was strait and it hasn't lost tension since.

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snip

 

One thing I don't understand is if the voltage supply to the warning light increases once the alternator is charging, how does it extinguish?

 

When you turn the ignition on current flows from the battery via the ignition switch, then through the warning lamp onto the warning lamp (D: usually) on the alternator. Inside the alternator it flows via the regulator and rotor to negative and thence back to the battery. The bulb as 12V (nominal) on the switch side and zero volts on the alternator side because the alternator connects it to negative.

 

As the alternator starts to charge the regulator and rotor start to be supplied from three internal diodes (field or aux diodes) but as the warning lamp is connected to the same place he curren tfrom the field diodes tries to "push back" up the warning lamp wire.

 

You end up with 12V from the battery pushing through the warning lamp in one direction and 12V form the field diodes in the other. 12V pushing against each other means no current can flow so the bulb goes out.

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